The preparation of cyclic esters of alpha-hydroxy carboxylic acids is an old and much studied process. Heretofore, the preparation has been conducted in two generally distinct batch steps involving first preparing an oligomer of the hydroxy carboxylic acid, i.e., a relatively short-chain condensation polymer thereof, then heating the polymer under reduced pressure to generate the desired cyclic ester. Gruter et al., U.S. Pat. No. 1,095,205 (1914); Lowe, U.S. Pat. No. 2,668,162 (1954); Bellis, U.S. Pat. No. 4,727,163 (1988); Muller, Ger. Patent Applications 36 32 103 and 37 08 915 (1988). Such processes spanning over 70 years of technology suffer in that they require hours of reaction time at high temperatures for the conversion of the polymeric intermediate to the cyclic ester. Further, the rather long residence times at the high temperatures employed often result in side reactions, leading, for example, to unwanted isomers, charring of the polymer and consequently difficult to handle reactor heels.
It will be noted the Muller German Applications, mentioned above, disclose continuous as well as batch operations for the depolymerization of lactic acid oligomers to lactide where the conversion takes several hours to complete. Muller states his process can be run batchwise, continuously or semicontinuously. These processes, however, depend strongly on greatly reduced pressures for the removal of the desired lactide from the reaction zone. Maintaining such low pressures is not only expensive on a commercial scale but does not solve the problems associated with long residence times. Also, product recovery under vacuum makes downstream processing cumbersome as many of these esters are solid at room temperature. The product obtained has large amounts of impurities, requiring solvent washing and recrystallization; and activated carbon treatment to remove impurities.
It is an object of this invention to provide an improved continuous gas-assisted process at low residence times and at high conversion rates for converting polymers (oligomers) of alpha-hydroxy carboxylic acids or their esters or salts to cyclic esters at high production rates. It is a particular object to provide such a process for producing lactide from the appropriate oligomer of lactic acid, ester of lactic acid or salt of lactic acid.